available devices using the i2c comms bus

[westaust55]

In searching around for i2c based modules that may be of interest and suitable for connection to the PICAXE microcontrollers, I found Devatech Ltd who are manufacturers of many of the modules sold by Rev-Ed and their distributors plus BASIC STAMP agents.
Of interest was that some modules such as the ultrasonic modules are available in both i2c comms as well as the discrete I/O connection modes typically sold by the microcontroller retail sellers. I do note that at least in Australia, in some cases, the i2c parts are far more expensive than the discrete I/O interfaced parts but there other advantages.
For those in the UK/Europe have a look at http://www.robot-electronics.co.uk/
For those in Australia have a look at http://www.robotparts.com.au/
Have any others here had any dealings with the above companies and in particular Robot Parts in Australia?
I would be interested to know what (other than EEPROM’s and RTC’s) are others connecting to their i2c bus in the way of sensing and interface modules.

 

[westaust55]

what are other doing using the i2c bus

I would be interested to know what (other than EEPROM’s and RTC’s) are others connecting to their i2c bus in the way of sensing and interface modules.

Is no one else doing anything using the i2c bus

 

[stocky]

yup - at this stage just "port expansion"

watch this space............a few things I'll be trying soon

 

[moxhamj]

Have you got the PCF8574 chip working yet?

 

[hax]

I have used the ADS1110 16 bit ADC, the speech module and some ultrasonic sensors in i2c mode. I much prefer using i2c over spi or any other method.

 

[hippy]

Quite timely that I've been spending the past few days writing I2C slave code. Once you have a micro which can operate as an I2C slave device the world is your oyster, limited only by the capabilities and cost of that micro and the effort you or some other kind soul is prepared to put into it.

Things which are not commonly available as I2C devices or simply hard to source can be implemented using a software I2C slave; TV interfacing, large I2C RAM, HDD / CF / SD interfaces, I2C to USB Slave ( ie serial to/from PC via USB ).

 

[BCJKiwi]

@ Westaust55,
Accelerometer, Compass, Clock, I/O expander (MCP23016 and now MCP23017), uAlfat-TF datalogging.

 

[westaust55]

Dr Acula,

Have you got the PCF8574 chip working yet?

Yes. I never had any problems with the i2c comms side. The issue was with the PCF8574 chips having high capacity (25mA) outputs only on LOW output.
To stay with conventional logic with HIGH = ON, I used interposing transistors between the PCF8574 and the LED's.
This also has the advantage that I can still drop in a 74HC595 shift register module I have built to operate the LED's (as the LED's were already)commoned to the 0V rail.

 

[nickelflipper]

Once you have a micro which can operate as an I2C slave device the world is your oyster, limited only by the capabilities and cost of that micro and the effort you or some other kind soul is prepared to put into it.

A slippery fish (oyster) it is. And for us less talented a bucketfull of patience, and much more than a few days.

 

[Michael V]

Capacitive sensor

Hey Westaus,

In my line of work i am interested in liquid level sensing, in particular non conductive liquid like oil and fuel. Capacitive level sensors are often used for liquid level, there are no moving parts or contacts, and they are highly reliable. The sensor itself is one steel tube inside another steel tube. I can't really say how the typical circuit works.

Upon searching I was pleased to learn that Analog devices have a capacitance to I2C chip: http://www.analog.com/en/prod/0,2877,AD7746,00.html . You can get it from Farnell, but it only comes as a TSSOP which i'm not even going to try to solder. But Sparkfun have a breakout board for $25US :
http://www.sparkfun.com/commerce/product_info.php?products_id=7918

It also has a built in temperature sensor, i'm not sure if that is also accessible by I2C.

I am very much hoping i can use two metal tubes, this chip, a picaxe 18X (or bigger), an xBee radio and a small solar panel, to build a wireless tank level measuring device.

Commercial systems that will do all this ( almost) won't give you change from $50,000 or more. A big part of that cost is sensors and wiring.
Commercial sensors won't give you any change from $1500. Here is an example
http://www.hectronic.com/en/refuelling/products/optilevel_hf.html?EMPHORASID=0060c78a6f0bb27b9cd188251ccf5d6f


So, if you can work out how i can get the AD7746 I2C to work with Picaxe then i would be very happy. Two concentric steel tubes plus rising liquid should give a linear response should it not? I would not know where to start, other than reverse engineering, but $25 is not a big investment.

I'm sure proportional capacitance sensing can be used for other things such as non metallic proximity sensors, range sensors, but liquid level is the obvious one for me.

Michael V

 

[westaust55]

i2c based level sensor for oil and fuel

In my line of work i am interested in liquid level sensing, in particular non conductive liquid like oil and fuel.

Oil and fuels tend to fall into the category of flammable and hazardous requiring equipment certified by a national testing body before they can be legally used in such locations.

Even for normal industrial applications, the electrodes are covered with a material like polyethylene or PTFE and which will be a couple of millimeters thick. Having used a few such commercial devices in the past for non-hazardous applications some are dual (concentric) electrodes, others use two parallel rods while some even use a single electrode measuring to the side of the tank.
In this day and age, you would be unlikely to find a capacitive type level sensor for non-hazardous materials for less than $1500. Many industrial instruments I have had to select for projects cost in the $2000 to $1000 range covering level, flow, etc

I believe (but cannot guarantee) that the capacitive change will be linear as you seek.

The AD7746 has a capacitive sensing range of: +/- 4.09 picoFarad
See the attached capacitor formula for some idea on how to design the electrodes.

Sorry, but while I have time to comemnt based on thinkgs I have knowledge upon (that is easy) my time is rather limited to delve in great detail into a new field such as instrument design.

 

[Michael V]

Thanks Westaus,
Yes, good safety warning. I would warn beginners (like me) against any electronic equipment attached to flammable fluids such as petrol (Gasoline). Treat it as per the warnings about messing with 240V unless you are a professional.

But - Lubricating oil has a "flash Point" above 180C, and although it is combustible, like paper and wood, it is not "flammable" like petrol and kero. You don't have red diamonds on a lube oil truck, only those with flammable liquids such as petrol. The cooking oil in your pantry (canola, olive) is actually more "flammable" than lube oil, and you only have to worry about that burning when you are deliberately heating it up on the stove or barbie.

I'm definitely leaving petrol to the experts, but lube oil (eg Hydraulic oil) is pretty safe, as long as i keep the voltage in picaxe range.

I take your point about insulation of electrodes, but, in the case of lube oil there is no benefit. That is one of the advantages of capacitive sensors, even if they get a lot of gunk on the metal surfaces they still measure liquid level OK.

If the fluid was corrosive, then yes, insulation is a must. For example, in a water tank a length of figure 8 speaker cable hanging vertically somewhere away from the sides might become a capacitive sensor. What that would be in pico or femto farads i don't know, but once i find out the permittivity of water vs the permittivity of air then i'll try out your calculation (thanks for that) and see if it fits under the 4.09 pico farads, and in a useful range above 4fF, which is said to be the resolution.

The AD7746 chip has 24 bits, That sounds like a lot.The data sheet lists level sensing, humidity sensing, purity sensing and flow meters in its list of applications. so it is going to be about electrode design, from which i will take tips from current designs, which are concentric tubes or a rod in a tube.

Capacitive level sensors have been in use way before there were dedicated chips. The release date on the AD7746 is 2005. Before chips they probably used some kind of multi op amp capacitance to analog converter with oscillating circuit with lots of noise and common mode rejection gizmos. This will be partly why the cost is so high. I like the idea of having a dedicated chip to handle all that, and there is obviously a market for it.

The AD7746 is an I2C device which, thanks to this great forum i think i can use now, and this is what your original question is about.

I'll buy one and let you know how i get on. May need more forum help if i can't figure it out. If the boys over at Sparkfun can do it then Picaxe can do it for sure.


Michael V